Differential Sticking (Cause of Stuck Pipe)
In normal overbalanced drilling conditions, a pressure differential exists between the mud column and the formation pressure. In situations where the formation pressure is significantly lower than that exerted by the mud column, the drillstring may be forced against the wellbore wall and, if the pressure difference is large enough and acts over a significantly large area of the pipe, the pipe may become differentially stuck.
The force required (F) exerted against the pipe may be calculated through:
F= Dp x A x Cof.